Thermoset composites have received widespread use in the aircraft industry as a structural material. This type of composite can provide a light weight structure of relatively high strength. To obtain a high strength structural bond with thermoset composites currently requires costly methods of bonding combined with mechanical fasteners.
One method of bonding thermoset composite structures together is to place thermoset resin film adhesive material between the surfaces to be bonded, followed by applying heat and pressure to the joint zone. This process is not very practical as it involves the application of relatively high heat (250.degree.-350.degree. F.) and pressure (25 psi. minimum) for extended periods of time (2 hours minimum at temperature). Thermal expansion can become a problem when large areas of structure are heated as required for this process. This is due to differing rates of thermal expansion of localized structure which causes thermal strain when the structure is cooled and can cause deformations. Another drawback is that thermoset adhesives can only be processed once and the bond is not reversible for repair or replacement of subsequently damaged sub-structure.
A second method currently used involves attaching together thermoset composite structure using mechanical fasteners combined with room temperature curing thermoset paste adhesive applied to joint surfaces. This method requires that the components being joined be pre-fitted and drilled for pilot holes, disassembled, cleaned and bond prepared. Adhesive is then applied to joint surfaces, the components are then reassembled, and the adhesive cured (usually 24 hours at room temperature). The next step is to drill and ream holes for each fastener and finally the fasteners are installed. This method is costly due to the use of mechanical fasteners and the labor intensive nature of the method.
An alternative to these processes is to use a film of thermoplastic adhesive between the thermoset composite structures being joined together. Since thermoplastic may be repeatedly melted and refused, the bonding process can be reversed as needed. Unfortunately, current thermoset resins will not chemically adhere with sufficient strength to suitable thermoplastic adhesive materials to meet aircraft requirements for joining structural components. Therefore this method is not practical in situations requiring high strength bonds.
Heating means for the bonding process are typically apparatus for induction heating or electrical resistance heating. Induction heating is adequate but difficult to apply for repairs or replacements performed in the field. Electrical resistance heaters, typically embedded in the bond, are more easily re-used for repair and replacement functions. The heating elements used as resistance heaters in the prior art have limitations, however, one limitation is the occurrence of hot spots and cold spots during the bonding process. The hot spots and cold spots are due to uneven heating provided by the heating element. The result is a degradation in the quality of the bond.